A vector network analyzer (VNA) is a useful instrument for many applications where electrical and/or microwave measurements, such as transmission and reflection properties, are needed. VNA's are usually used where the electrical signals have a high frequency, ranging from (but not limited to) 10 kHz to 100 GHz. Since a VNA can be used to measure complex impedances of circuits at high frequencies, VNAs can be found in many electronic and radio frequency (RF) laboratories, as well as in chip/microwave device or system manufacturing facilities.
A VNA can apply a stimulus sine wave to a device under test (DUT) and perform a series of measurements and calculations. VNAs are often used to characterize two-port networks such as amplifiers and filters, but they can be used on networks with an arbitrary number of ports. A two-port VNA can measure both reflected signals from the DUT and transmitted signals through the DUT. Additionally, the VNA can calculate S-parameters and other related parameters for that DUT. The VNA can repeat this procedure using different frequencies and/or power levels to measure the desired characteristics of the DUT.
The basic architecture of the VNA includes a signal generator, a test set, one or more receivers and a display. A traditional VNA test set 100, as shown in FIG. 1, may include four ports (110, 120, 130 and 140) which may be connected, for example to the DUT ports 150. Each of the test unit ports may be connected to a source transmitter and requires two directional couplers which are connected to two receivers for measuring the reference signal (i.e. R1, R2, R3 and R4) and the received signals (i.e. A, B, C and D). Therefore, according to the prior art solution two receivers are required for each test port. The traditional VNA further includes a number of switches and couplers, such as couplers 115, 117, 125, 127, 135, 137, 145 and 147 located on each branch of the VNA test set 100. The couplers are configured to sample, measure and direct the transmit signal (forward) and the return signals (backward direction) at the VNA (for each direction a single coupler is needed).
The receivers and the transmitters at the VNA are synchronized according to methods known in the field. The testing may be performed simultaneously on all the VNA's ports or separately and alternately at each port.
As illustrated in FIG. 1 the traditional VNA is a complex device which typically occupies a large space, includes multiple elements (such as switching elements connectors and couplers) and is expensive. Moreover, some of the elements are mechanical elements (i.e. coaxial switches) that must be frequently switched, resulting in the decrease of the traditional VNA's reliability.
The prior art solutions that are currently used to overcome such problems, rely on either utilizing many couplers and receivers as shown in FIG. 1 or makes use of a balun as shown in FIG. 2. The conventional basic measurement bridge 200 of FIG. 2 comprises a plurality of resistors 211 and a balun 210.
An example of a directional bridge and a balun scheme is illustrated in U.S. Pat. No. 4,962,359 to Dunsmore entitled “Dual directional bridge and balun used as reflectometer test set”. According to Dunsmore there is provided a test set for use in measuring S-parameters with a network analyzer includes a first directional bridge, a second directional bridge and a single balun with two outputs mounted in an RF housing. A test signal from an RF signal source is transmitted through the test set to a device under test. The first directional bridge separates a signal from the device under test and the test signal, and provides the signal from the device under test to a coupled port. The second directional bridge separates the test signal and the signal from the device under test and provides the test signal to a reference port. The balun includes a coaxial transmission line with its outer conductor grounded at an intermediate location to define first and second balun sections. Ferrite beads are mounted on each of the balun sections. The ends of the first and second balun sections are coupled to the first and second directional bridges, respectively.
The disadvantage of applying the scheme disclosed by this publication is that it requires either magnetic components or large transmission line components to achieve broadband operation i.e. coupler or a balun for separating an input test signal to a device under test and a signal from the device under test.
It would therefore be desirable to provide an improved, cheap and compact device without requiring use of couplers (e.g. balun).
The term “Vector Network analyzer (VNA)” as used herein and through the specification and claims should be understood to encompass an electrical device used to generate and transmit RF signals and to measure the ratios between the received RF signals and the transmitted one. Those relations represent the reflection and transmission coefficients of the tested port.
The term “S-parameters” as used herein and through the specification and claims should be understood to encompass scattering parameters, e.g. the set of reflection and transmission coefficients of a system from each port to the other. Two ports “S-parameters” typically include 4 terms:
Port1 and port2 reflection coefficients (2 Terms)
Port1 to port2 and port2 to port1 transmission coefficients (2 Terms)
The term “T Parameters” as used herein and through the specification and claims should be understood to encompass scattering transfer parameters, which are another representation of the S-parameters, in which concatenation of 2-ports is translated into matrix multiplication of the corresponding T-parameter matrices of the corresponding 2-port elements.
The term “Antenna” as used herein and through the specification and claims should be understood to encompass an RF element used to transfer electrical RF signal (voltage and current) to propagating wave (electrical & magnetic fields).
The term bridge′ as used herein and through the specification and claims should be understood to encompass a type of electrical circuit in which two circuit branches are “bridged” by a third branch connected between the first two branches at some intermediate point along them.
The term “balun” as used herein and through the specification and claims should be understood to encompass a device that joins a balanced line (one that has two conductors, with equal currents in opposite directions, such as a twisted pair cable) to an unbalanced line (one that has just one conductor and a ground, such as a coaxial cable).